The concept of closing the vents of the crawlspace or building a crawlspace with no vents in new construction is relatively new in the construction industry and has only been an option in the last ten years or so. Building codes have been evolving to address these new crawlspace setups.

The concept is seal the crawlspace up with heavy duty plastic barrier both on the ground and up the side walls. The idea is to suppress ground moisture and to keep humid outside air from coming into the space. It is recognized that closing a crawlspace with no drying action is not sufficient. (Although that has been done by contractors as well, most agree that it leads to problems unless a drying tool is used.) In the conditioned crawlspace setup, the HVAC or air conditioning from in the house is being pumped into the crawlspace and used to dry the air just like it would inside the house upstairs.

While the theory is somewhat sound and it seems to be working in some homes, in reality, the applications are leading to issues and concerns. Here are some of the primary issues:

The conditioned space has no fresh air. Over time, some of these crawlspaces are beginning to have musty odors from beneath the plastic. As the entire crawlspace is sealed off with no outside ventilation, the odors have nowhere to go and are permeating up into the living space of the house.

The conditioned air is being pushed into the crawlspace in various ways, sometimes with a pipe fan and other times just with a register vent. The air from upstairs will continue to be pushed into the crawlspace to dry the crawlspace. However, if that air is not escaping out the sides of the closed crawlspace or being exhausted out via a fan to the outside, there will be a point where there is so much positive pressure into the crawlspace that it will no longer accept new dry air. Keep in mind that the crawlspace moisture doesn’t reduce unless you have drier air going through the crawlspace or if the wetter air is going through a cold air return from the HVAC effectively acting as a dehumidifier. Once it is pressurized to a certain point, the air that is supposed to go into the crawlspace will now bypass it as it is unable to push through to the crawlspace. There have been examples where this has occurred and the humidity levels were not coming down as they should have and a dehumidifier had to be added as an additional source to create a drying action within the crawlspace.

There is very little consistency in the drying coming from the HVAC system as it is not being measured by the crawlspace needs it is running based on the home’s HVAC needs. The drying action is also reliant on the HVAC system running at all, so for homes that are unoccupied during certain times of the year with HVAC systems turned down or off, the crawlspace is left unprotected. This is often a problem in foreclosed homes or homes in new construction where a crawlspace doesn’t yet have electricity and HVAC running.

Additional concerns arise when there is a potential radon concern as any radon will be trapped in the crawlspace with no chance to escape.

Homes with any gas furnaces or other gas appliances located in the crawlspace are also of concern as any gas leaks or combustion issues will also be trapped inside the crawlspace.

Most homes with conditioned crawlspaces are starting to see issues about five years after installation of this setup. If you are experiencing this, see how the ATMOX System may be able to help in your situation to re-vent or partially open a closed crawlspace.